A common issue when upgrading operating systems is losing your Windows 7 profile. Even if you think your data will be safe, always back up your data before an upgrade.
User profiles are personal settings that establish how the system interface looks and works. Sounds, screensavers, desktop backgrounds and more can be set. Occasionally, Windows won’t read the profile correctly, any users may create a new one as instructed.
The first thing you should do is check to see if the profile is still there, by going into C:\Users/username. If it is there, you can follow Microsoft’s guide for restoring a corrupted profile. Another option is to try restoring your machine to point before the account disappeared.
If this fails, don’t worry, your data can still be recovered. The most important thing is not to write anything onto the hard drive. Provided the required data isn’t overwritten, there is a high chance of a success.